The present invention relates to coating materials comprising
A) a hydroxyl-containing polymer or a mixture of hydroxyl-containing polymers, and PA1 B) one or more blocked di- and/or polyisocyanates at least some of whose isocyanate groups are blocked with dialkyl malonate. PA1 b1) from 30 to 85% by weight of one or more dialkyl malonates of the general formula (I) ##STR1## PA1 and/or one or more dialkyl malonates of the general formula (II) ##STR2## PA1 where PA1 and PA1 b2) from 70 to 15% by weight of one or more dialkyl malonates other than component (b1), PA1 b1) from 30 to 85% by weight, preferably from 50 to 70% by weight, of one or more dialkyl malonates of the general formula (I) ##STR3## PA1 and/or one or more dialkyl malonates of the general formula (II) ##STR4## PA1 where PA1 and PA1 b2) from 70 to 15% by weight, preferably from 50 to 30% by weight, of one or more dialkyl malonates other than component (b1),
The present invention further relates to the preparation and use of the coating materials and to the preparation of the blocked polyisocyanates used in the coating materials of the invention and to the use thereof in topcoats and clearcoats.
It is known to use blocked polyisocyanates as crosslinkers for stoving enamels. Stoving enamels for industrial use, such as, for example, as topcoat, basecoat or surfacer for automotive OEM finishing, are baked preferably at temperatures from 120 to 140.degree. C. The objective is to provide one-component coating materials which can be cured at very low baking temperatures and which nevertheless are of sufficient stability on storage at room temperature. For this purpose it is common to use coating materials based on hydroxyl-containing binders and on blocked isocyanates as crosslinkers.
It is also known that isocyanates blocked with diethyl malonate react with the OH groups of acrylate copolymers even at very low temperatures of about 80.degree. C. Isocyanates blocked with diethyl malonate, however, are of very low solubility, and/or the stability of their solutions on storage is inadequate.
In order to solve this problem, DE-A-42 04 518 proposed nonaqueous coating materials in accordance with the preamble of the main claim. The use described therein of other blocking agents together with malonic diesters, however, lowers the curing rate and impairs the color number and yellowing resistance of the coatings. Furthermore, the oligomerization of the isocyanate component results in an increase in viscosity of the crosslinker component and thus in a lowering of the attainable solids content of the coating materials.
In addition, DE-A-195 19 396 discloses (cyclo)aliphatic polyisocyanates blocked totally or partly with malonic esters, and their use in coating compositions, where the malonic ester contains at least one tert-butyl group. In comparison to polyisocyanates blocked with diethyl malonate, these polyisocyanates have a deblocking temperature which is reduced by from 10 to 20.degree. C. The use of malonic esters having at least one tert-butyl group as blocking agents, however, has the disadvantage that the stability of the coating material prepared there on storage is inadequate even at room temperature.
DE-C-195 29 124, moreover, discloses coating compositions in connection with which it is essential to the invention that the binder used comprises a mixture of a styrene-containing acrylate copolymer and a styrene-free acrylate copolymer. As crosslinkers, these coating compositions include one or more free or blocked polyisocyanates. In addition to a large number of other blocking agents, dialkyl malonates are also mentioned in DE-C-195 29 124, such as, for example, inter alia, diethyl, diisopropyl, and di-tert-butyl malonate, diethyl malonate being used with preference among these dialkyl malonates.
Furthermore, U.S. Pat. No. 4,677,180 discloses coating compositions which in addition to a hydroxyl-functional binder include blocked isocyanates in which the isocyanate groups are blocked with C1 to C12 alkyl malonates. The only example of suitable malonates mentioned in this document, however, is diethyl malonate. A further disadvantage is that these coating compositions known from U.S. Pat. No. 4,677,180 mandatorily require the addition of a low molecular mass compound having at least 2 OH groups per molecule as a stabilizer.
JP-A-2-242867 describes a basecoat/clearcoat process in which nonaqueous transparent topcoats comprising (A) a hydroxyl-containing synthetic resin, (B) an amino resin, and (C) a blocked polyisocyanate are applied, components (B) and (C) being selected such that the temperature at which a chemical reaction ensues between (A) and (C) should be not more than 20.degree. C. below and not more than 50.degree. C. above the temperature at which a chemical reaction ensues between (A) and (B).
As blocking agents for the preparation of component (C) it is preferred to use aliphatic monoalcohols, oximes, and caprolactams. However, suitable blocking agents mentioned also include malonic esters. The transparent topcoats described in JP-A-2-242 867 provide paint systems which in respect in particular of their resistance to organic solvents and acids, their gloss, their transparency, and their resistance to yellowing are deserving of improvement.
Finally, DE-A-23 42 603 describes a process for preparing polyisocyanates blocked with dialkyl malonates having 1 to 4 carbon atoms in the alcohol residue, and the use of the blocked isocyanates in coating materials. Further details on the composition of the coating materials and their use, however, are not contained in D-A-23 42 603.
The present invention is therefore based on the object of providing coating materials where both the finished one-component coating material and the crosslinker used have an adequate stability on storage and where the coating materials are at the same time curable at very low temperatures. This means that in the case of storage of the finished one-component coating material at room temperature for 6 months a viscosity increase of less than 10% occurs or that on storage of the crosslinker for prolonged periods of up to 2 years or more there is no significant viscosity increase.
Furthermore, the coating materials should ensure only a minimal yellowing tendency on baking and also good acid resistance, scratch resistance and high gloss of the resulting coatings. Finally, the coating materials should be suitable for producing multicoat paint systems and in particular should meet the requirements commonly imposed in the sector of automotive OEM finishing.
This object is surprisingly achieved by the coating materials of the types specified at the outset, wherein the isocyanate groups of component (B) are blocked using a mixture of dialkyl malonates comprising
R.sup.1, R.sup.2 and R.sup.3 are each identical or different aliphatic and/or cycloaliphatic and/or araliphatic secondary alkyl radicals having at least 3 carbon atoms, and PA2 R.sup.4 is an aliphatic, cycloaliphatic, aromatic or araliphatic hydrocarbon radical, PA2 R.sup.1, R.sup.2 and R.sup.3 are each identical or different aliphatic and/or cycloaliphatic and/or araliphatic secondary alkyl radicals having at least 3 carbon atoms, and PA2 R.sup.4 is an aliphatic, cycloaliphatic, aromatic or araliphatic hydrocarbon radical,
the sum of the weight fractions of components (b1) and (b2) being in each case 100% by weight.
The present invention further relates to the preparation and use of the coating materials and to the preparation of the blocked polyisocyanates used in the coating materials of the invention and to the use thereof in topcoats and clearcoats.
It is surprising and could not have been foreseen that the partial use of dialkyl esters of malonic acid with secondary alcohols instead of dimethyl malonate and/or diethyl malonate as blocking agents for polyisocyanates leads to coating materials which are notable for improved yellowing resistance of the resulting coatings (especially to yellowing occurring due to high baking temperatures and/or due to long baking times). A further advantage is that the use of malonic diesters of secondary alcohols improves the solubility and compatibility of the blocked polyisocyanate.
Another advantage is that the coating materials of the invention can be cured at low baking temperatures to coatings having good acid resistance and scratch resistance. Moreover, the blocked polyisocyanates used in accordance with the invention are simple and inexpensive to prepare. Finally, the coating materials of the invention are suitable for producing multicoat paint systems, meet the requirements commonly imposed in the sector of automotive OEM finishing, and can also be processed well with a low level (e.g., less than 50% by weight) of organic solvents.